136 SPECIAL PHYSIOLOGY. 



will even restore the irritability after it has been suspended. Con- 

 tractility, as already stated, is wholly lost in atrophied muscle. 



Phenomena accompanying Muscular Contraction. 



Certain important phenomena accompany those changes of form 

 and condition in the muscles, which constitute their so-called contrac- 

 tion. In the first place, there is a sound produced by the contraction 

 of muscles, which may be easily heard by placing one finger so as to 

 close the ear, resting the elbow upon a table, and then contracting the 

 muscles of the fore-arm. This sound has been well compared to the 

 rumbling noise of distant carriages, and is called the muscular sound ; 

 it is probably owing to the friction of the contracting fibres against 

 each other ; its vibrations are said to be from thirty-two to thirty -six 

 per second. 



Another phenomena accompanying muscular contraction is the pro- 

 duction of heat. The fact may be shown by direct experiments with 

 the thermometer ; but the exact amount of elevation of temperature 

 can be more accurately measured by means of a thermo-electric appa- 

 ratus, of which the contracting muscle forms a part. 



If a metal ring be made of a semicircle of copper wire, and of another of iron 

 wire, soldered together at their ends, and if one of the points of junction be 

 made hotter or colder than the other, then thermo-electric currents, i. e., cur- 

 rents of electricity developed by heat, are produced in the compound metallic 

 ring. By introducing a needle galvanometer in the circuit of the ring, the 

 direction and force of such currents can be measured for each degree of unequal 

 temperature in the two points of junction. 



A needle galvanometer consists of a magnetized needle, suspended horizon- 

 tally by a single fibre of silk, and placed under cover of glass, means being pro- 

 vided for passing a current of electricity in its neighborhood at will. A circu- 

 lar card or disc, marked with degrees, and fixed beneath the needle, accurately 

 registers any movement which takes place in the latter. 



Now, when a current of electricity is made to pass, in any definite direction, 

 near such a magnetized needle, the latter is deflected, or turned to one side or 

 the other ; the wire through which the electrical current passes, itself acts like 

 a magnet, and tends, by virtue of mutual attractions and repulsions, to cause 

 the needle to stand at right angles to it. The direction of the deflection depends 

 on the direction of the current, and the amount of deflection on the force of the 

 current. If the observer looks down upon a galvanometer, with the north pole 

 of the magnetized needle turned from him, and a current of electricity be passed 

 along a neighboring wire, above the needle, also in a direction from him, the 

 needle will deviate to the left hand ; but if the current were passing under the 

 needle in the same direction, the needle would deviate to the right hand. If 

 the current passes towards the observer above the needle, the needle is deflected 

 to the right, and if below it, to the left. Now it is obvious that if the wire, 

 along which the current is made to pass, be bent into an oblong horizontal 

 loop, within which the needle is suspended, so that the current passes from 

 the spectator above the needle, and returns towards him under it, the force, which 

 causes the needle to deflect towards the left hand, is doubled ; because the de- 

 parting current above the needle, and the returning current below it, have both 

 a tendency to make the needle deflect in the same direction, i. e. , to the left. 

 By covering, and so insulating the wire, and by multiplying its departing and 

 returning bends, by coiling it up an immense number of times into the required 

 oblong loop, within which the needle may be suspended, the deflecting force is 

 still more powerfully increased ; and in thHs way, with a coil of very fine wire, 

 many thousands of yards, nay, even some miles in length, exceedingly feeble 



